1. Technical Field
The present invention relates to a displacement detector. For example, the present invention relates to a laser beam interference type displacement detector.
2. Related Art
A laser beam interference type displacement detector has been known (For example, See JP-A-4-270920). As shown in FIG. 11, the related-art laser beam interference type displacement detector 200 includes: a scale 210; and a detection head portion 220 capable of relatively moving in the length measuring direction of this scale 210, for detecting a displacement with respect to the scale 210.
The scale 210 has a reflection type diffraction grating 211 arranged in the longitudinal direction which is the length measuring direction. The detecting head portion 220 includes a light emitting and receiving portion 230 and an optical device unit portion 260.
The light emitting and receiving portion 230 includes: a light source 240 for emitting a laser beam (L31); and a light receiving portion 250 for receiving interference light which has been reflected and diffracted by the scale 210.
The optical device unit portion 260 includes: a beam splitter 270; a first mirror 281; a second mirror 282; a first corner cube 291; and a second corner cube 292. The beam splitter 270 divides light (L31) sent from the light source 240. The first mirror 281 reflects (L34) one beam of light (L32), which has been divided by the beam splitter 270, to the scale 210. The second mirror 282 reflects (L35) the other beam of light (L33), which has been divided by the beam splitter 270, to the scale 210. The first corner cube 291 retroreflects (L38) one beam of reflected and diffracted light (L36), which is sent from the scale 210, to the scale 210. The second corner cube 292 retroreflects (L39) the other beam of reflected and diffracted light (L37), which is sent from the scale 210, to the scale 210.
For the explanations made later, the terminology is defined as follows. The direction of length measuring of the scale 210 is the x-axis direction, the direction of the diffraction grating groove is the y-axis, and the direction of the normal line of the scale 210 is the z-axis direction as shown in FIG. 12.
In this case, in order to make the displacement detector 200 compact, the light emitting and receiving portion 230, the first mirror 281 and the second mirror 282 are arranged on one side of the center line (x-axis) along the length measuring direction of the scale 210, and the first and the second corner cube 291, 292 are arranged on the other side.
That is, beams of light incident upon the scale 210 via the first mirror 281 and the second mirror 282 are obliquely incident with respect to the diffraction grating grooves. In other words, a so-called conical diffraction is composed.
In this structure, light L31 emitted from the light source 240 is divided by the beam splitter 270 (L32 and L33 in FIG. 11). Then, the thus divided beams of light are reflected by the first and second mirrors 281,282 (L34 and L35 in FIG. 11) and then incident upon the scale 210 and reflected and diffracted. The beams of reflected and diffracted light (L36 and L37 in FIG. 11) sent from the scale 210 are retroreflected by the first and the second corner cubes 291, 292 (L38 and L39 in FIG. 11) and reflected and diffracted again by the scale 210 (L40 and L41). These beams of reflected and diffracted light L40, L41 are reflected on the first and the second mirror 281, 282 (L42, L43) and synthesized by the beam splitter 270 and received by the light receiving portion 250.
When a light receiving signal outputted from the light receiving portion 250 is processed by a predetermined signal processing procedure, a relative displacement between the scale 210 and the detecting head portion 220 is detected.
When the reflected and diffracted light sent from the scale 210 is retroreflected as described above, the four times multiplied optical signal can be obtained, and the resolving power can be enhanced.
However, the above structure is a so-called conical diffraction in which beams of light (L34, L35) incident upon the scale 210 are obliquely incident with respect to the direction of the diffraction grating groove and beams of diffracted light (L36, L37) are obliquely incident with respect to the direction of the diffraction grating groove.
In this case, in general, the condition of conical diffraction can be expressed by the following expression.βP·cos ε·(sin α±sin β)=m·λβ=sin−1{(mλ/cos ε·P)−sin α}  [Expression 1]
In this case, λ is a wave-length of light (for example, 635 nm), m is a degree of the diffraction, and P is a pitch of the grating. The angle α formed between the incident light (L34), which is projected on the x-z face, and the normal line, the angle β formed between the diffracted light (L36), which is projected on x-z plane, and the normal line, and the angle (conical angle) ε formed between the incident light (L34) and the x-z plane are shown in FIG. 12.
According to the above expression, in the case where a relative posture between the scale 210 and the detection head portion 220 is changed when the scale 210 yaws round the vertical axis (z-axis) or the scale 210 rolls round the x-axis, the incident angle α and the conical angle ε are changed. Therefore, the diffraction angle β is changed.
FIG. 13 is a view showing an optical path in the case where the scale 210 yaws. FIG. 13 also shows a portion of the structure of the displacement detector 200.
As shown in FIG. 13, when the diffraction angle β of the diffracted light L37, which is generated when the incident light L35 sent from the second reflection mirror 282 is diffracted, is shifted, the optical path of light is shifted. Therefore, the recursive optical length (L37+L39) fluctuates and the positional information deviates. When a position at which the retroreflected and diffracted light is incident upon the beam splitter is shifted (that is, when a position at which L43 is incident upon the beam splitter 270 is shifted) an intensity of the interference signal is decreased and the detection accuracy is lowered.